Vast quantities of acid mine drainage (AMD) are contained within the now abandoned underground mining voids on the Witwatersrand in South Africa and other locations. AMD arises from the contact of mineral sulphides, e.g. iron sulphide or pyrite, with water and oxygen, which chemically generates a dilute sulphuric acid. AMD is thus typified as a low pH, corrosive aqueous substance having a high content of dissolved metal salts.
Although a host of chemical processes contribute to the formation of acid mine drainage, pyrite oxidation is by far the greatest contributor. A general equation for pyrite oxidation is:2FeS2(s)+7O2(g)+2H2O(l)→2Fe2+(aq)+4SO42−(aq)+4H+(aq)  (1)
The oxidation of sulphide to sulphate solubilises iron II (ferrous iron), which is then subsequently oxidized to iron III (ferric iron) according to the equation:4Fe2+(aq)+O2(g)+4H+(aq)→4Fe3+(aq)+2H2O(l)  (2)
The oxidation of sulphide to sulphate and the oxidation of ferrous iron to ferric iron can either occur chemically spontaneously or it can be catalyzed by microorganisms that derive energy from the oxidation reaction. The ferric iron produced can further oxidize additional pyrite and itself form additional ferrous iron according to the reaction:FeS2(s)+14Fe3+(aq)+8H2O(l)→15Fe2+(aq)+2SO42−(aq)+16H+(aq)  (3)
The nett effect of these reactions is to increase the concentration of hydrogen ions in solution, thereby lowering the pH and maintaining the solubility of ferric iron.
Being an acidic medium, AMD is capable of dissolving and mobilizing other toxic metal salts found in tailings dumps, rock and reef dumps, and the underground cavities in which the AMD is formed, e.g. salts of copper, nickel, zinc, manganese and aluminum. It will be appreciated that AMD will thus contain a variety of dissolved metal salts which would be harmful if allowed to escape to the environment.
Water levels within underground mining basins, including those in the Witwatersrand area in South Africa, have accumulated AMD and have continued to rise over the years. Levels are now very high and AMD is overflowing in certain areas, e.g. on the West Rand in South Africa. In the large central Witwatersrand basin, AMD is widely expected to overflow from the mining cavities in approximately the next two to four years.
It is thus an aim of this invention to provide a means of alleviating these AMD problems.